The present invention relates generally to a continuously-variable speed transmission for motor vehicles and more particularly to a transmission for motor vehicles employing a belt type continuously-variable change ratio device, designated as "a belt device" hereinafter. The belt drive device includes a first variable pitch pulley and a second variable pitch pulley being associated with an input shaft and an output shaft, respectively and rotatively connected.
Each of the variable pitch pulleys of the belt drive device comprises a fixed flange secured to the associated input or output shaft, a movable flange axially and slidably mounted on the other shaft, an endless belt interconnecting said variable pitch pulleys and means capable of regulating the distance between the movable flange and the fixed flange in order to change the radial position of the endless belt which is held between the fixed flange and the movable flange so that the ratio of the revolving speed of the output shaft to that of the input shaft, that is, the speed change ratio, is changed. Belt drive devices of this type are well-known. Incorporation of a belt drive device of this type into a transmission for motor vehicles allows continuously-variable change of the revolving ratio between the input shaft and the output shaft, namely, the speed change ratio, automatically but not necessarily in response to the driver's demands or road conditions.
However, in applying such a belt drive device as described hereinbefore to a transmission for motor vehicles, additional provision of a reversing changeover gear mechanism and a reduction gear is required since the speed change ratio of the belt drive device, in general, is variable within a range of 2.0 to 0.5. The input shaft of the belt drive device and the output shaft of a prime mover are interconnected with a coupling device which transmits rotation without converting the torque, such as a fluid coupling or a clutch, and the output shaft of the belt drive device is connected to the driving axle of a motor vehicle through a conventional final reduction gear mechanism of the hypoid gear type.
Employment of a planetary gear mechanism in the reversing changeover mechanism is desirable in order to provide a compact transmission for motor vehicles. A simple planetary gear mechanism of the single planetary pinion type is unsuitable to the reversing changeover mechanism for the transmission for motor vehicles, owing to a great difference between the gear ratio for normal rotation and that for reverse rotation, which is inherent to the planetary gear mechanism of a single planetary pinion type.
A planetary gear mechanism of the double planetary pinion type is quite favorable, since the gear ratio is 1.0 both for normal rotation and for reverse rotation. However, when the planetary gear mechanism of the double planetary pinion type is coupled directly to the belt drive device as described hereinbefore, a reduction gear ratio of approximately 8 is required of the final reduction gear mechanism. A final reduction gear mechanism of the hypoid gear type with such a great reduction gear ratio has never been available and is not suitable for motor vehicles. Accordingly, machine designers having ordinary skill will incorporate a reduction gear of the necessary reduction gear ratio into a reversing changeover planetary gear mechanism of the double planetary pinion type in order to attain a combination of the belt drive device and a conventional final reduction gear of the hypoid gear type. In this case, it is usual to arrange and interlock the mechanisms in a sequence of a fluid coupling or a clutch, a belt drive device, reversing changeover planetary gear mechanism of the double planetary pinion type, a reduction gear and a conventional final reduction gear mechanism. In a transmission system for motor vehicles thus constructed, torque transmission efficiency is likely to be reduced since the torque of the prime mover is transmitted to the conventional final reduction gear mechanism through two sets of gear trains of a planetary gear mechanism and a reduction gear. At the same time, the high-speed operation of a planetary gear mechanism of the double planetary pinion type affects the durability of the planetary gear mechanism disadvantageously.
What is needed is a continuously variable transmission for motor vehicles which has high torque transmission efficiency, simple construction and high durability.